With the development of industrial technology, demands for high efficiency, high performance, low cost, and safety have been further increased, and the usage environment of a steel structure has become severe. Thus, a design engineer is desired to rationally design a structure from mutually contradictory standpoints of high performance, economically efficiency, and safety.
However, there are often cases where a structure being supposed to be designed safely is damaged earlier than expected. In the case of a steel structure, it is said recently that “fatigue” directly or indirectly causes 80% or more of the damage as a result of dramatically improving the performance of a material.
Fatigue design is carried based on an S-N curve, in which break (crack initiation) life of a test piece under constant load amplitude is simply expressed as a function of stress amplitude, without considering an actual physical phenomenon, with only focusing attention on that cumulative stress frequency distribution applied on the structure comes to a certain fatigue damage level or less. Fatigue design using the S-N curve is effective to some extent as an empirical rule being feedback on actual use, but information about the size of a crack and the like cannot be obtained and there are many cases where the fatigue design using the S-N curve is disabled in a new style structure.
In damage analysis, on the other hand, there are many cases where crack propagation life assessment is carried out with postulating the existence of an initial crack based on the fracture mechanics. In this case, the supposed initial crack is appropriately adjusted so as to fit with damage, so that the fatigue design behaves as if it would function properly. In actual fact, however, the fatigue design only takes measures against individuals by reducing operating stress by increasing thickness and the like, and any fundamental treatment cannot be adopted. Therefore, it is desired to establish a fatigue design method by which the initiation and the propagation of a crack can be discussed on an equal footing.
The inventor has pointed out a contradiction that the fatigue design using the S-N curve assumes that a crack of a certain size suddenly appears in a sound area and a fatigue phenomenon belonging to the category of stable fracture is dealt as unstable fracture so far. As a result of study for a long period of time, the inventor found out that the initiation and the propagation of a crack could be assessed with a single parameter before anyone else in the world, and developed theory that a continuous crack growth curve could be assessed from a crack of a size zero by this parameter.
The inventor estimates fatigue life of a crack propagating through a first grain as described in Non-patent document 1 with assuming a case wherein a certain amplitude load is cyclically applied on the basis of the simple assumption that a tensile plastic zone appears during a loading process and a compressive plastic zone appears during an unloading process and the size of a region which both of the zones overlap determine crack propagation rate.
Non-patent document 1: “Fatigue Life Assessment For Steel Structures” written by Masahiro Toyosada and Toshio Niwa, published by Kyoritsu Shuppan Co., Ltd. on Dec. 25, 2001, pages 182 to 186